Influence of Corrosion Scale on CO₂ Corrosion of Marine Gas Pipeline

CO₂ corrosion is one of the main reasons for leakage failure of offshore natural gas pipelines, practice showed in the area where pipeline corrosion occurred, if the corrosion scale layer in the area was uniformly covered, the corrosion phenomenon under the scale layer was significantly less than that in the area where the corrosion scale layer was not complete.In order to determine the reason for this phenomenon, electron scanning microscopy (SEM) and X-ray diffraction (XRD) were used to analyze the surface microstructure and chemical composition of the corrosion scale layers at the corrosion failure sites of pipelines.Electrochemical technologies were used to study the effect of scale layers on CO₂ corrosion of X65 steel. The results show that the uniform corrosion rate of X65 steel in CO₂ declined in the existence of FeS and FeCO₃ corrosion scale layers; when FeCO₃ corrosion film completely covered on the surface of metal substrate, the possibility of pitting of metal substrate would be significantly lower;on the contrary, if FeCO₃ corrosion film covering on the metal substrate surface was loose and porous or incomplete, a tiny galvanic cell structure would be formed on the metal substrate surface, thus leading to an increased tendency of pitting on the metal substrate surface.